1. Field of the Invention
The present invention relates to an apparatus and method for transmitting control information in a mobile communication system, and more particularly, to an apparatus and method for transmitting uplink control information in an orthogonal frequency division multiple access communication system (OFDMA).
2. Description of the Related Art
Current mobile communication systems are developing toward the 4th generation which provides ultra high-speed multimedia services. The 4th generation system is the end result of the 1st generation analog mobile communication system, the 2nd generation digital mobile communication system, and the 3rd generation mobile communication system providing high-speed multimedia services such as the IMT-2000. This 4th generation mobile communication system provides access to a number of networks including a satellite network, a wireless local area network (WLAN), an Internet, etc., in one mobile terminal. In effect, the 4th generation mobile communication system can support all services of voice services, video services, multimedia services, Internet data services, voice mail services, instant message services, etc., by using one mobile terminal. The 4th generation system strives for a transmission rate of 20 Mbps and mainly employs an orthogonal frequency division multiplexing (OFDM) scheme to provide high-speed multimedia services.
Herein, the OFDM scheme implies a digital modulation scheme for multiplexing a plurality of orthogonal carrier signals. Such an OFDM scheme is used for dividing a single data stream into several low-speed streams and simultaneously transmitting the low-speed streams by using several sub-carriers having low transmission rates.
A multiple access scheme based on the OFDM scheme is referred to as an orthogonal frequency division multiple access (OFDMA) scheme. The OFDMA scheme denotes a scheme in which sub-carriers in one OFDM symbol are distributed to a plurality of users (i.e., a plurality of subscriber terminals). A communication system based on such an OFDMA scheme includes additional physical channels for transmitting uplink control information. These physical channels include a channel quality information (CQI) channel, an ACK/NACK channel, and a coefficient for multi-input multi-output (MIMO) channel.
Hereinafter, a structure of a transmitter 10/receiver 20 for transmitting/receiving the uplink control information in the conventional OFDMA communication system will be described with reference to the accompanying drawings.
FIG. 1 is a schematic block diagram illustrating an internal structure of the transmitter 10. The transmitter 10 includes a binary channel encoder 11, a modulator 12, and an inverse fast fourier transform (IFFT) unit 13.
If information data bits of uplink control information are generated, the binary channel encoder 11 receives the information data bits and outputs a codeword to the modulator 12 by using binary channel codes (e.g., (20,5) block codes).
Herein, the modulator 12 includes a coherent modulator or a differential modulator. The modulator 12 receives the codeword output from the binary channel encoder 11, finds transmit symbols corresponding to the codeword based on a coherent modulation scheme or a differential modulation scheme, and then, outputs the transmit symbol to the IFFT unit 13. Herein, the modulator may employ a predetermined modulation scheme (e.g., a quadrature phase shift keying (QPSK) scheme or a differential quadrature phase shift keying (DQPSK) scheme).
The IFFT unit 13 receives the transmit symbol from the modulator 12, performs IFFT with respect to the transmit symbol, and then outputs the transmit symbol.
Hereinafter, an internal structure of the receiver 20 will be schematically described with reference to FIG. 2. The receiver 20 includes a fast fourier transform (FFT) unit 23, a demodulator 22, and a binary channel decoder 21. If a receive signal is input from the transmitter 10, the FFT unit 23 performs FFT with respect to the receive signal and outputs a receive symbol to the demodulator 22.
The demodulator 22 may include a coherent demodulator or a differential demodulator correspondingly to the modulator of the transmitter 10. The demodulator 22 receives the receive symbol output from the FFT unit 23 and finds a soft decision value of the receive symbol based on a demodulation scheme corresponding to the modulation scheme of the transmitter (e.g., a coherent demodulation scheme or a differential demodulation scheme).
The binary channel decoder 21 receives the soft decision value found in the demodulator 22, determines which the transmitted codeword is, and outputs information data bits corresponding to the decided codeword.
Although the uplink control information transmitted/received through the transmitter 10/the receiver 20 having the above described structure occupies a small portion in the overall communication service, it is necessary to ensure high reliability with respect to the transmission of the uplink control information because the uplink control information is very important in communication system operation. However, generally speaking, a physical channel for transmitting the uplink control information is allocated a small number of frequency-time axis resources to reduce overhead rates. Accordingly, it may be advantageous to employ a transmission method different from that for channels such as traffic channels transmitting much information and allocated many resources.
The conventional technique employs a transmission method in which binary channel encoding is combined with coherent modulation or differential modulation as described above in order to transmit the uplink control information.
However, if the uplink control information is transmitted in the transmission method using a small number of frequency-time axis resources, the number of errors may increase, thereby lowering the stability of communication system operation. In other words, the uplink control information may be transmitted with insufficient pilot tones. As a result, channel estimation performance is degraded, thereby degrading the performance of the coherent modulation/demodulation. Herein, if the number of pilot tones is increased in consideration of only channel estimation, the number of data tones may be excessively insufficient. In addition, if the binary channel encoding is performed separately from the modulation, the separation may prevent the achievement of optimized performance. Further, if many frequency-time axis resources are used for the transmission of the uplink control information to raise system stability, overhead rates may increase, thereby reducing the throughput of a communication system.